Hello

?

Cooking the tubes.

?

It was necessary to modify the tube polarization system. As shown previously, the HT power supply is floating relative to ground. This trick allows the cathode voltage (the filaments) to be modified relative to ground and as the grids are galvanically connected to ground, it allows the grid - cathode voltage to be adjusted. This is what happens in reception where the tubes are blocked with a grid - cathode voltage of -60V.

?

To be able to positively polarize the grid relative to the cathode, in operational operation, the original diagram does not allow it. This diagram must therefore be modified. The positive polarization voltage of the grid can only come from an external source; it is shown as a green battery on the diagram

?

?

Obviously this involves some work including drilling the back panel to get the two wires out of the green source. I put in two female banana plugs and a bridge to place a short circuit in place of the auxiliary power supply to find the original diagram (0 V polarization).

?

I remind you of the objective: to cook the anodes to red under a low anode voltage and a high current. All this to degas the tubes. It will certainly also be useful to improve the linearity by increasing the quiescent current which is possible since in France we are limited to 500 W HF (against the almost 2 kW that the PA can output).

?

The first tests are done with a 230/230 transformer which unfortunately only makes 400 VA (it must not exceed 1.7 A on the secondary). The polarization is done from a 48 V transformer and a Graetz bridge rectifier.

?

·???????? With a polarization at 0 the plate voltage is at 688 volts and the quiescent current (since there is no HF in excitation) is 30 mA

·???????? With a polarization at 18 V DC (but it is rectified bi-alternation) the HT is 611 V, the plate current 300 mA (therefore 183 W) and the grid current (DC) 85 mA

·???????? With a polarization at 23 V DC the HT is at 593 V the plate current at 400 mA (237 W) the grid current 260 mA and I am at the limit of the transformer 1.6 A RMS

?

That is not enough to make the plates red.

?

I switched to a single 3-500Z tube. As a precaution, there is a load at the input and output but the system seems quite stable.

?

Now it works

?

Oddly enough, it only takes 20 Ω in series in the primary to bring the filament voltage back to 5.0 V. I plugged the hole caused by the missing lamp so as not to short-circuit the ventilation that I forced to the max. I placed a glass plate above the lamp box in order to leave the ventilation as when the cover is in place but being able to keep an eye on the anode of the tube (otherwise, you can't see the tubes).

?

Two tests that give almost the same result: the anode in its lower part is dark red.

·???????? Bi-alternation rectified voltage on the grid Vg DC = 38 V Ig DC 110 mA plate current 390 mA under 592 V (231 W) secondary consumption: 359 VA

·???????? Bi-alternation rectified voltage + 6300 ?F capacitor so, basically DC on the grid Vg DC = 41.9 V Ig DC 120 mA plate current 400 mA under 584 V (234 W) secondary consumption: 389 VA

?

I am limited by the 400 VA max of the 230/230 transformer supplying the voltage doubler.

?

The measured values ??are questionable; I expected more difference when I added the capacitor. I am surprised that with 234 W on the anode, we are already in the dark red; the tube is given for 500 W of anode dissipation. I don't dare remove the anode radiator which also serves as a connector; I suppose it is this which prevents the entire anode from being red, from top to bottom.

?

How long should I cook it for?

1-2 hrs max...per tube.

?

Alright. Then I'll do the second 3-500Z valve.

Then I'll do the full voltage tests with the Drake's HT transformer

--

F1AMM

Fran?ois